1. Field of the Invention
The present invention relates to a method for manufacturing a high-pressure discharge lamp.
2. Description of the Related Art
General high-pressure discharge lamps such as an ultra high-pressure mercury lamp comprise an elongated cylindrical quartz glass tube which is comprised of a hollow spherical section, straight tube sections formed at both ends of the hollow spherical section, and electrode assemblies, each of which has an electrode that is partly embedded in an associated straight tube section and is opposite the electrode of the other electrode assembly in the spherical section. Each of the electrode assemblies has a molybdenum foil, a tungsten electrode shaft welded at one end of the molybdenum foil, and an external lead wire welded at the other end of the molybdenum foil. Then, the molybdenum foil, as well as part of the tungsten electrode and part of the external lead wire, welded at both ends of the molybdenum foil, are encapsulated in glass of the associated straight tube section. Mercury, an inert gas, and a halogen gas are enclosed in the space of the spherical section.
For welding the tungsten electrode shaft to the molybdenum foil, the tungsten electrode and molybdenum foil are sandwiched in layers between welding electrodes of a resistance welding machine, pressurized between the welding electrodes, and applied with a voltage for welding.
Another method of directly welding a tungsten electrode to a molybdenum foil, as mentioned above, involves forming a platinum thin film on a tungsten electrode shaft at which the molybdenum foil is welded, and resistively welding the platinum thin film to the molybdenum foil (see Paragraphs [0009] to [0011], and FIGS. 2 and 3 of JP-6-223783-A). Another method involves interposing a blazing member such as platinum, tantalum or the like between overlapping portions of a molybdenum foil and an internal lead wire which are scheduled to be welded, and performing plasma arc welding (see Paragraph [0038] and FIG. 3 of JP-10-334789-A). As described in the aforementioned JP-6-223783-A and JP-10-334789-A, a halogen lamp is generally manufactured by sandwiching a platinum foil between a molybdenum foil and a tungsten electrode, and melting the platinum foil during welding in order to weld the tungsten electrode to the molybdenum foil.
However, in the method of welding a tungsten electrode shaft to a molybdenum foil using a resistance welding machine, the tungsten electrode shaft can be bent, broken, or collapsed at a point applied with pressure, resulting in failure of the tungsten electrode itself, and eccentricity (see FIG. 1) of the electrodes which oppose each other within the lamp. In addition, even the application of pressure can be difficult for a tungsten electrode shaft having a small diameter.
Also, the melted tungsten electrode and molybdenum foil can stick to the welding rods (welding electrodes) of the resistance welding machine, and the welding rods can significantly wear out, so that frequent maintenance is required for the welding machine, making the welding process unsuitable for automatization.
Further, since the resistance welding encounters difficulties in the application of pressure to a small point, the molybdenum foil must be welded to the tungsten electrode over a wide area. Thus, when the molybdenum foil with the tungsten electrode welded thereto is sealed with glass, a large crack occurs in a portion of the glass sealed with the molybdenum foil, possibly resulting in defective air-tight sealing of the lamp due to the growth of the crack, and a bursting of the lamp when it is turned on.
Notably, the method of indirectly welding a tungsten electrode and a molybdenum foil with platinum or the like interposed therebetween, as described in the aforementioned JP-6-223783-A and JP-10-334789-A, entails an increased number of manufacturing steps and a higher cost.